Digital filters have varied applications, one of which is for speech synthesis. Speech synthesizers sometimes use a partial autocorrelation (PARCOR) lattice filter structure to model the human vocal tract. One well known PARCOR lattice structure utilizes multiple stages, each stage comprising two multipliers and two adders as shown in U.S. Pat. No. 3,662,115 entitled "Audio Response Apparatus Using Partial Autocorrelation Techniques". PARCOR coefficients can be related to boundary value equations for the transmitted and reflected sound pressure wave in an acoustic tube model of the vocal tract. A sound pressure wave in this acoustic tube model comprises a transmitted wave and a plurality of reflected waves, the sum of which produces a plurality of impulses, each of which is separated by an exponential amplitude decay. These impulses are generated by the vocal cords of a human. The vocal cords function similar to a relaxation oscillator by bursting open and passing an impulse of air from the lungs into the vocal tract. When the pressure on both sides of the vocal cords is equalized, the force of the neck muscles cause the vocal cords to close. This action of the vocal cords produces a type of speech sound termed voiced. An example of voiced speech would be any vowel. Another type of speech sound is termed unvoiced. The `s` sound in `hiss` is an example of unvoiced speech. PARCOR lattice structures having a minimum of four stages for unvoiced speech and eight stages for voiced speech are generally required for quality synthesis.
Typical disadvantages with digital filters using the two multiplier PARCOR lattice filter structure include problems with either hardware complexity, control complexity, calculation speed, coefficient and interstage precision or circuit die size, or any combination of the above. For example, some digital filters which implement a PARCOR lattice structure have aperiodic control signals which are complex to implement. Other implementations have used either fully parallel or pipelined conventional multipliers which are inefficient in both power and size although speed efficient. Previously, when any one of the above problems has been overcome, the performance of the PARCOR lattice structure decreases with respect to at least one of the other problems. For example, although time multiplexing of PARCOR lattice filter structures has been suggested in the prior art, the previously mentioned problems still exist.